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Lugging an engine

by CAR magazine on 10/02/2011

Comments: 0

I’ve often heard it said that it’s bad to employ large throttle openings at low engine speeds in a high gear. Surely a modern engine is designed to take all the stresses that occur in day-to-day driving?

SIMON McQUEEN – By e-mail

The rotating and reciprocating parts of an engine is always working against some resistance, unless the engine is idling. The engine experiences a greater resistance in the higher gears than in the lower gears, because it has to supply a larger percentage of whatever torque the driving situation demands. For example, if a driving situation demands 240 N.m at the gearbox output shaft and the gearbox is in fifth gear with a 0,8 ratio then the engine has to deliver 240/0,8 = 300 N.m, but in a 1:1 fourth gear the engine’s portion is 240 N.m. In the same way a 1,6:1 third gear would result in only 150 N.m coming from the engine. In first and second these torque demands decrease even more.

The rotational speed of the engine also plays a role in determining the stress being experienced . At low revs the effect of any stress is more severe because the engine experiences the loading for a longer time. At high revs any load is only being applied for milliseconds, but at low revs it may take larger fractions of a second to dissipate.